Strategy to improve synaptic behavior of ion-actuated synaptic transistors—the use of ion blocking layer to improve state retention

Synaptic transistors (STs) with a gate/electrolyte/channel stack, where mobile ions are electrically driven across the solid electrolyte, have been considered as analog weight elements for neuromorphic computing. The current (ID) between the source and drain in the ST is analogously updated by gate voltage (VG) pulses, enabling high pattern recognition accuracy in neuromorphic systems; however, the governing physical mechanisms of the ST are not fully understood yet. Our previous physics-based simulation study showed that ion movement in the electrolyte, rather than the electrochemical reactions that occur in the channel, plays an important role in switching. In this study, we experimentally explore the properties of the HfOx electrolyte and show that by tuning the density of oxygen vacancies, it can assume the dual role of electrolyte and channel. We demonstrate analog synaptic behavior using a novel ST with a two-layer stack of CuOx/HfOx, where the CuOx is the gate and Cu ion reservoir, and the HfOx is the electrolyte and channel. To improve state retention and linearity, we introduce a Cu ion transport barrier in the form of a dense and stoichiometric Al2O3 layer. The CuOx/Al2O3/HfOx exhibits excellent state retention and improved potentiation and depression response. Energy dispersive spectroscopy mapping following potentiation confirms the role of the Al2O3 layer in confining the Cu ions in the HfOx layer. We also show that a two-step programming scheme can further enhance synaptic response and demonstrate high recognition accuracy on the Fashion-MNIST dataset in simulation.

electrolyte are driven toward the channel in the vertical direction.Various mobile ions such as Li +17,19 , O [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]21 , H +22,23 , and Cu +24,25 have been explored.The broadly accepted picture is that the switching takes place only when the ions reach the channel and directly dope or convert the valence state of the channel's atoms. Thus, theextent of the ions intercalated into the channel, which is related to the electrochemical potential applied to the gate stack by the V G bias, analogously increases or decreases the current between the source and drain (I D ), corresponding to potentiation or depression, respectively. Wehave recently shown that electrolyte charging also induces a charge in the channel material and that, in some cases, this may be the preferred mechanism 26 .Moreover, our physicsbased simulation results of CuO x -gate/HfO x -electrolyte/WO x -channel stacks have shown that Cu intercalation into the WO x channel leads to Cu plating, loss of linearity, and enhanced degradation 18 .
In this study, we focus on the HfO x layer and use a CuO x /HfO x ST to demonstrate that by tuning the density of oxygen vacancies, it can also assume the role of the conducting channel making the WO x layer redundant.We also show that an ultrathin Al 2 O 3 film, inserted between the CuO x and HfO x layers, acts as an ion barrier that confines the Cu ions to the HfO x layer and improves the state retention and linearity.
The three-terminal STs composed of all CMOS compatible electrodes and layers were formed on a Si substrate with a thermally grown 100 nm-thick SiO 2 wafer.First, as shown in Fig. 1a, source (S) and drain (D) contacts were patterned and deposited by sputtering with W target at a power of 50 W.The patterning was performed via conventional photolithography, developing, and lift-off processes.Afterwards, a 5 nm-thick HfO x electrolyte with a length (width) of 50 (150) μm was deposited by sputtering with HfO 2 target at a power of 100 W under Ar and O 2 gas flows at the rate of 25 and 5 sccm, respectively.Next, a 360 nm-thick CuO x gate electrode was deposited by sputtering with Cu target at 100 W using Ar and O 2 gases at rates of 27 and 3 sccm, respectively.Finally, a W capping layer was deposited to prevent unwanted copper oxidation.The fabricated CuO x /HfO x ST was analyzed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), as shown in Fig. 1b,c.As mentioned in our recent publications 25 , to limit the number of mobile Cu ions participating in I D switching, the CuO x gate electrode was adopted instead of the previously proposed Cu gate.As shown in Fig. 1d, the intensity of the Cu-O bonding at binding energies of 943 and 948 eV was detected in the measured Cu 2p peak 27 .Further, a non-stoichiometric HfO x layer comprising both Hf-Hf metal and Hf-O oxide bonds were observed (Fig. 1e) 28 .As discussed below, we consider the oxygen vacancies as facilitators of copper ions transport.
Figure 2 shows the importance of the HfO x stoichiometry and the role of oxygen vacancies in determining the electrolyte's properties.Figure 2a shows the response for a device using the previously developed HfO x 25 , sputtered using 30 sccm Ar only.Although a low initial I D of 28 nA was obtained, the potentiation pulses of V G = + 3 V and a pulse width of 100 ms flood the entire layer with Cu ions, thus reaching the maximum current within the first pulse.Applying the read voltage of 0.5 V to D and 0 V to S results in a current of about a 1.4 mA.Since the Cu ions transport is expected to be assisted by oxygen vacancies, we introduced oxygen flow and used sccm of 20:10 Ar to O 2 , respectively.The response to potentiation pulses is shown in Fig. 2b.Reduced oxygen vacancies render the HfO x insulating to electron and Cu ion transport.Following a few pulses with low I D , the  www.nature.com/scientificreports/current abruptly jumps to its maximum value.It means that the high field concentrated across the insulating HfO x layer led to permanent oxide breakdown (inset to Fig. 2b), resulting in low gate controllability.The response of an optimized device using an HfO x sputtered under 25:5 Ar:O 2 flow is shown in Fig. 2c,d.Figure 2c, shows the response to potentiation by 10 pulses of 100 ms width and V G values between − 3 V and 3 V.We note that V G = 3 V initiates a linear potentiation response with the Cu ions transport into the 5 nm HfO x layer being well controlled.An extended potentiation/depression response using 50 pulses is shown in Fig. 2d.While not being an ideal response, it clearly demonstrates the importance of controlling the density of oxygen vacancies to achieve well behaved gate controlled uniform Cu-ion migration throughout the electrolyte.
Based on the synaptic behavior in Fig. 2d, the linearity factor, α, was calculated by the following equation 29 : where, G MAX and G MIN are conductance at the maximum and minimum I D state, respectively, and ω is an internal variable which ranges from 0 to 1.Moreover, α is equal to 1 in the case of the ideal synaptic behavior.Based on these equations, linearity of potentiation (α pot ) of 1.51 was achieved during potentiation in CuO x /HfO x ST.A hint for the process causing the sublinear potentiation can be found in the depression response to negative V G pulses with amplitude of − 1 V and pulse width of 100 ms.The first pulse results in more than 50% reduction of the current with the response to the following pulses saturating quickly.The resulting a non-linear response has a linearity of depression (α dep ) of − 1.28.Based on the results of Fig. 2d, we postulated that the nonlinearity is associated with facile Cu ions transport out of the HfO x layer.
To improve the retention of the Cu ions within the HfO x layer we introduced a 2 nm Al 2 O 3 film between CuO x and HfO x .We used atomic layer deposition to ensure a relatively dense and stoichiometric layer that would serve as a partial barrier for Cu ion transport.The use of ultrathin Al 2 O 3 film is important to avoid introducing extra ion resistance that may hamper the dynamic range.The deposition was done at a chamber temperature of 200 °C using trimethylaluminum and water sources.The approximately 2 nm-thick Al 2 O 3 layer was deposited at a deposition rate of approximately 1.1 Å/cycle. Figure 3a shows an high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the CuO x /Al 2 O 3 /HfO x stack where energy dispersive X-ray spectroscopy (EDS) mapping of Hf (Fig. 3b) and of Al (Fig. 3c) confirm the layers' position.Figure 3d shows Cu mapping following 100 potentiation pulses of the CuO x /Al 2 O 3 /HfO x ST.We note a uniform distribution within the HfO x layer and no Cu signal within the Al 2 O 3 one.The uniformity in the HfO x layer is a testament to the successful morphology and stoichiometry supporting uniform Cu ion injection.Most importantly, the absence of Cu signal in the Al 2 O 3 layer supports the notion that it acts as a barrier where the particles can only go through but not reside within.
Figure 4a  While it should be possible to enhance the device architecture further, we chose to demonstrate the effect of using two gate pulses instead of just one.Essentially, a short pulse of opposite polarity with a width of 50 ms was added to the conventional single V G pulse (see inset to Fig. 4b).Naturally, adding the opposite polarity pulse To show that the Al 2 O 3 layer confines the Cu ions to the HfO x layer and acts as partial barrier to Cu ion transport, we tested the state retention during analog switching (Fig. 5).As the top of Fig. 5a shows, the test procedure has a basic block consisting of 10 potentiation pulses followed by a sequence of read pulses for 100 s.This block is then repeated several times.In the context of Fig. 2, we mentioned that the sublinear potentiation is probably associated with poor state retention and fast discharge during potentiation.Figure 5a clearly shows that the CuO x /HfO x ST has poor retention.In contrast, the response of the CuO x /Al 2 O 3 /HfO x ST (Fig. 5b) shows a stable state-retention allowing us also to test the stability during depression.Namely, the Al 2 O 3 layer is acting as a barrier confining the Cu ions to the HfO x layer and thus preventing state discharge during read operation.
Finally, we built a multilayer neural network comprising the input, hidden, and output layers based on backpropagation algorithms, as demonstrated in Fig. 5c.The input, first hidden, second hidden, and output layers were composed of 784, 250, 125, and 10 neurons, respectively.The signals were transferred from input neurons to output neurons through synaptic weights, which served as the fabricated STs in this study.The recognition accuracy on the Fashion-MNIST dataset was evaluated using an IBM analog hardware-acceleration simulator kit (AIHWKIT) with a learning rate of 0.01 30 .When highly asymmetric synaptic behavior owing to abrupt I D drop during depression obtained from a CuO x /HfO x ST stack was used, low recognition accuracy of approximately 67% was achieved, which corresponds to an error rate of approximately 33% at 20 iterations, as shown in Fig. 5d.On the other hand, the near-ideal recognition accuracy of approximately 93% with significantly lower error rate of 7% was obtained by exploiting a CuO x /Al 2 O 3 /HfO x ST.
To conclude, we introduced analog switching based on a structure that avoids the use of WO x channel layer, letting the HfO x take the role of both the electrolyte and the conducting channel.For this, the stoichiometry of the HfO x had to be fine-tuned to support stable Cu ion transport as well as electron transport (probably via trap-assisted tunneling) 31 .A sweet spot in terms of oxygen vacancies was found for the process involving 25:5 Ar:O 2 sccm flow.However, a 5 nm HfO x that is directly attached to the Cu ion reservoir (CuO x ) exhibits poor state retention as the Cu ions are easily pulled back into the CuO x layer.This resulted in a sub-linear potentiation response and, more pronouncedly, a 50% drop during the first depression pulse (Fig. 2).To mitigate the facile pullback of Cu ions, we use an ALD process to introduce a relatively dense and stoichiometric Al 2 O 3 layer between the HfO x and the CuO x .EDS studies (Fig. 3) showed that the Al 2 O 3 layer acts as a barrier confining the Cu ions to the HfO x layer.Consequently, the CuO x /Al 2 O 3 /HfO x stack shows improved state retention and a better linear response (Figs.4,5).Lastly, to test the quality of the CuO x /Al 2 O 3 /HfO x ST response, we implemented it to simulate pattern recognition using IBM AIHWKIT, resulting in an error rate as low as 3%.

Figure 1 .
Figure 1.(a) Schematic diagram, (b) cross-sectional TEM image, and (c) XPS depth profiling of the fabricated CuO x /HfO x ST.(d) Cu 2p and (e) Hf 4f peak intensities of CuO x and HfO x layers, respectively.

Figure 2 .
Figure 2. (a,b) Impact of Ar and O 2 gas flow rates during HfO x deposition on the I D response of the CuO x / HfO x ST.(c) I D response of the CuO x /HfO x ST, with optimized HfO x stoichiometry, as a function of polarity and amplitude of V G .(d) The update curve of I D in the ST employing optimized HfO x electrolyte layer.
shows the analog synaptic behavior of the CuO x /Al 2 O 3 /HfO x ST to be compared with that of the CuO x /HfO x ST shown in Fig. 2d.The insertion of the Al 2 O 3 layer improves the potentiation and somewhat mitigates the initial current drop in the early depression phase.The degree of the I D change per pulse became relatively constant except for the first pulse during the depression stage, resulting in α dep of − 0.48.
Figure 3. (a) HAADF-STEM image of the CuO x /Al 2 O 3 /HfO x stack.(b,c) EDS mapping image showing the position of the HfO x layer through (b) the Hf signal and of the Al 2 O 3 layer through (c) the Al signal.(d) EDS mapping of CuO x /Al 2 O 3 /HfO x ST following 100 potentiation pulses showing the Al 2 O 3 layer being free of Cu ions which are being confined to the HfO x layer.

Figure 4 . 7 Figure 5 .
Figure 4.The update curve of I D in the CuO x /Al 2 O 3 /HfO x ST programmed by (a) conventional single V G pulse method and (b) two-step programming pulse scheme.As a result, the degree of the I D per V G pulse became uniform even in depression.